MIKE ROSS wrote in a message to Roy J. Tellason:

 MR> Hi, Roy.

 MR> I don't know if you received my reply in sci.electronics.repair as
 MR> the bbs here has been having problems for the past week or so.

No,  I haven't seen anything from you in there as of earlier today...

 MR> Here's the original reply:

 MR> "Roy J. Tellason" bravely wrote to "All" (18
Oct 03  20:47:54)  ---
 MR> on the heady topic of "ESR meter?"

 MR> I've been testing my homebrew adaptor for a dmm to measure ESR. It
 MR> uses a common NE555 timer (any mfgr). This is what it looks like: 

 MR> :   ESR adaptor for DMM
 MR> :   -------------------
 MR> :
 MR> :            o +9V *regulated
 MR> :   ,--------|
 MR> :   |        |
 MR> :   |  .-----------.
 MR> :   `--:4    8    3:---------+--3K9----+----o-------> probe
 MR> :      :           :         |         |    |
 MR> :      :   "555"  6:--.     10K        |    |    set DMM to
 MR> :      :  timer    :  |      |       200R (DMM)  200mV AC range :  
 MR> :     1    2:--+------'         |    |
 MR> :      `-----------'  |                |    |
 MR> :            |       === 3n3F*         `----o-------> probe
 MR> :           _|_       |                    _|_
 MR> :           ///      _|_                   ///
 MR> :                    ///

Not bad except that it's slightly screwed up in the middle,  I'm guessing
tabs where there should be spaces?  Something like that?

 MR> The operating principle of this circuit depends on the DMM being
 MR> able to measure "AC" up to about 100K Hz.

I'm not sure what any of mine will do in that regard.  I guess it's easy
enough to find out,  if I could get to my scope and find some place to set
it up in this mess.

 MR> Many DMM's are able to do so accurately on the lowest voltage 
 MR> range since this bypasses the attenuator but you will have to try 
 MR> and see.

Yep.

 MR> The output voltage is about 200milliVolts so it won't bias 
 MR> junctions (except Germanium).

I don't expect I'll encounter much of that these days.  :-)

 MR> It will thus work well in-circuit as long as the impedances are 
 MR> much greater than 200 ohms (which is often the case).

That seems to be a pretty common feature of a lot of ESR-measuring stuff.

 MR> The 3n3F capacitor (0.0033uF) sets the output frequency to about
 MR> 30KHz which is a medium value that will measure capacitors well
 MR> between about 10uF to about 100uF.

Seems like a reasonable range to me.

 MR> Larger capacitors may show a false reading because the inner layers
 MR> are shunted by the outer ones. So for large capacitors of over
 MR> 220uF the 3n3F cap should be replaced by a 100nF cap (0.1uF). The
 MR> lower test frequency will measure the inner layers better this way.

Or wire 'em both in there with a dipswitch or something.

 MR> With capacitors of about 4u7F (4.7uF) and less the capacitive
 MR> reactance becomes a significant contribution at 30KHz so a
 MR> capacitance of 1nF should be substituted for the 3n3F timing
 MR> capacitor.

Likewise.

 MR> The problem of course is that the DMM now has to measure
 MR> AC at 100KHz and not all can.

Yeah.  I can see where it would be possible to do some more circuitry to
give an indication,  even if it were something as simple as a unity-gain
buffer with rectification,  so that you're only trying to read DC at the
end of it.  A freq of 100k ain't _that_ far out there.

 MR> The only critical part is the voltage supply which must be a
 MR> regulated 9 volts. Otherwise you will have to re-calculate the
 MR> output divisor ratio. Sorry about it but aside from that this
 MR> simple adaptor works well. 

Don't have any 9v regulators but there are *lots* of 7805 (and even 78L05
which I'm sure would suffice for this) around here...

 MR> Discharge capacitors first!!! I guess a couple of protection diodes
 MR> on the input might help in case one forgets.

Yeah,  I tend to add stuff like that to things I build.  A bridge rectifier
gets .001 caps across it,  and regulators get a couple of ceramic and
tantalum caps in addition to the main filter,  plus a couple of protection
diodes.  I've salvaged enough of the silly things,  might as well use 'em!

I collected a bit of other ESR meter info from other responses to that
initial post,  but I've snagged so darn *much* info off the 'net in the
past few days that I'm currently in the process of trying to get it
organized,  before I go out and get some more.  Nice to have a local HTML
tree here...    

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